Well depending on the purpose of the water tank. I know in my house the water tank in the attic is the main water supply, thus if you turn on your taps, thats the water from that tank.
Everytime you use water, flushing the bog, brushing your teeth etc causes the level to go down, causing a valve to open to top up the tank, in other words its not really sitting for very long.
Water from your tap can be used to water your plants - no problem, unless you have a really really bad water supply etc.
Don't let all the talk of super pure this, filtered that etc worry you, in many places tap water is perfect. Or when in doubt there is always bottles of mineral water. Unless you're growing in a strict hydro system, don't worry about it.